For weight training intensity is gauged by:
a) amount of exercises performed
b) volume/weight being used
c) sets and reps

If you want to get all precise and pro about it, you can calculate your 1 Rep Max, which is basically the maximum amount of weight you can lift in just one rep. After calculating your 1 rep max, you can figure out your weight range for the amount of reps you plan to do.
Click here to calculate your 1 rep max.
You can also refer to my article on Training Types, to better help you reach your specific goals, whether it is strength, power, endurance, or hypertrophy (increasing muscle size).

Time: How long you exercise (per day)

For cardiovascular exercise, really speaking, we should be doing at least 10 minutes a day, everyday – this is the bare minimum. However, ideally, we should be aiming for 30-60 minutes per cardio session, deepening on your fitness level.

For strength training, it varies, depending on what you are doing in the weight room; for example, if you are doing a total body workout, you may be at the gym for an hour, but if you are doing a 3-4 day split, you might only be in the gym for 10-30 minutes. Just remember, whatever you are doing, it should be beastly! Make sure all your workouts are intense (for your fitness level) and make sure to push yourself out of that comfort zone!

Type: Specific kind of exercise being performed.

For cardiovascular exercise this is basically any form of exercise or movement that gets your heart rate up, so that could be anything from running, to swimming, to Kundalini Yoga, to dancing.

The F.I.T.T Principle is a great way to create a pretty solid fitness schedule and routine. It also allows for easy modification when your body starts to adapt, thus allowing us to dodge that pesky plateau stage.
By modifying only one of the four principles, you can avoid boredom and plateauing; for example, changing the intensity of your strength training routine by increasing the amount of reps, or the number of sets, or changing the amount of days a week you train, or the types of exercises being used.
Pretty helpful tool for the novice and the pro!